Inspection of fluids such as foodstuffs for the presence of extraneous material has frequently relied upon visual or tactile procedures which limit processing rates and which are subject to operator error and inattention.
Previously, as in U.S. Pat. No. 4,208,915, it has been proposed to compress discrete, solid, form-retaining food products, e.g., frozen hamburgers, against a flexible membrane which can assume the shape of the surface of the solid and transmit thereto, from a transducer, ultrasonic sound which is used for inspection of the solid item in a manner similar to that in which ultrasonic sound has previously been used for inspection of metal parts and castings, welds in metal and the like.
The use of ultrasonic sound energy for examination of the human body for diagnostic procedures in medicine is also known, e.g., "Diagnostic Ultrasonics: Principles and Use of Instruments" by W. N. McDicken, published 1976 by John Wiley & Sons.
However, none of these previously proposed techniques is suitable for the continuous detection of extraneous materials in fluids such as processed foodstuffs.